Selections from van Ooststroom's key are given below. The key is not binary (e.g, 3—3*—3**, 16—16*—16**). But it isn't even ternary; e.g., for leaves of E. sericeus which are sericeo-villous on both sides section 39 must be consulted for features not conflicting with those in the key (under 15) [the blue arrows, lines]. Nonetheless, the text itself does clearly handle the differentiae without problems. My impression is that although many botanists reference van Ooststroom, few have actually bothered with the text — thus taxa with a pubescent upper surface are determined as not E. sericeus (e.g., in Texas/New Mexico as E. nuttallianus). All taxa under 14 have distichous leaves. Why van Ooststroom hedges on this feature is not entirely clear — for E. arenicola he gives "distichous or secund," but the type collection only has distichous leaves. I have noted that distichous leaves frequently end up appearing to be secund as a mounting artefact.
